1. Field of Invention
This invention is directed to a method and apparatus for a cross-connected Time Division Multiplexed (TDM) digital audio bus for computer systems or stand alone recording/mixing systems. More specifically, the invention is directed toward the connection of multiple nodes to form a data network for processing and recording data in a digital audio system.
2. Discussion of the Related Art
Central conference controllers and Time Division Multiplex Networks are presently used in telephone systems. In these networks, remote stations consisting of telephones and data processing apparatus are connected to a central processing system. The processor at the central processing system receives data from each station, processes the data, and redistributes the processed data to all stations on the network. Examples of conference controllers using a central processing station are disclosed in U.S. Pat. Nos: 4,271,502, 4,224,688 and 5,027,347.
U.S. Pat. No. 4,646,288, entitled "Multiline Accumulator/Multiplexer", issued to Shumway discloses a multiplexer that can be used as a central processing control station for conferencing. In the system disclosed by Shumway, the data received from Time Division Multiplexed channels is summed in an accumulator. Shumway discloses a buffer that is alternatively connected to a TDM bus and to the accumulator to receive data from the TDM bus and provide the data to the accumulator. The data summed in the accumulator is then distributed. The accumulator/multiplexer is a simplex system that provides for limited one way communication between a client and the TDM bus.
U.S. Pat. No. 4,575,845, entitled "Time Division Multiplex Conferencer", issued to L. Baranyai et al, discloses a simplex (one-way) digital audio mixing system. The system disclosed by Baranyai performs similarly to that of Shumway described above, except that Baranyai does not disclose storage (i.e. a buffer) of the digital audio data received from the TDM bus prior to providing the data to the accumulator. This lack of storage allows only one opportunity per TDM time frame to acquire data and sum it with other incoming data. Since the data flow in Baranyai is simplex, a second controller is required for full duplex communication.
The prior art systems include several limitations that restrict their use in a digital audio system. In conferencing systems of the prior art, all signals received over the TDM bus have the same gain and are summed before the data is presented to the client. These conference networks of the prior art are limited in that a large number of channels slows the response time of the network. Since there is not buffering, or buffering only prior to the accumulator, the data order is fixed to the same order as the time slots across the bus, and the client has essentially no control over the order in which it will see the data.
The prior art is also limited in that a master clock of the TDM bus is synchronous with a division of time slots on the bus and the sample rate of the data. In these systems, the data transfer rate, the length of each time slot, and the sample rate are directly tied to the master clock rate. These systems do not have flexibility in terms of the number of time slots provided versus sample rate. Once the sample rate is chosen in these systems, the master clock is fixed, or if the master clock is chosen, the sample rate is fixed.